Phycobilins are the major accessory photosynthetic pigments of cyanobacteria. They are found in structures call phycobilisomes which lie on the surface of the thylakoid membranes and may comprise over 50% of the cell protein. Phycobilins also serve as a large reserve of amino nitrogen in these organisms, and the proteins are degraded during nitrogen starvation. This is particularly important in nitrogen-fixing cyanobacteria, since it supports synthesis of nitrogenase and the other proteins needed for nitrogen fixation and heterocyst development. Our long term goals for research on phycobilins are to identify and characterize the genes for phycobilins and related proteins which constitute phycobilisomes and to understand how the genes are regulated in relation to nitrogen metabolism in nitrogen-fixing cyanobacteria. This work is one part of more broadly based studies we are conducting on the molecular biology of nitrogen metabolism in cyanobacteria. The specific aims of the studies proposed here are to identify and characterize the genes for the major phycobilins of Anabaena variabilis 29413 and to study their regulation in relation to different states of nitrogen metabolism, especially nitrogen starvation. We will isolate genes from our Ch30- Anabaena library with heterologous probes and/or with synthetic oligonucleotides. The genes will be sequenced, and their organization in the genome will be determined. We will use hybridization techniques, pulse labeling, and spectroscopic analysis to study regulation of phycobilin gene labeling, and spectroscopic analysis to study regulation of phycobilin gene expression. Several different nitrogen sources, as well as nitrogen starvation, will be examined for their effect on mRNA and protein levels.